As shown in FIG. 9, generally speaking, in a Raman amplification system, when guiding an optical signal 2 output from a light transmitting station 1 to a light receiving station 4 through a light transmission path 3, such as an optical fiber, an optical amplifier 5 is arranged at some midpoint in the light transmission path 3 or at an end thereof to optically amplify the optical signal so that the optical signal 2, which has been attenuated in the light transmission path 3, may attain the requisite optical signal level for reception by the light receiving station 4. However, in some cases, the optical amplifier 5 is provided at the forward end or both ends of the light transmission path 3.
In the light transmitting station 1, electrical information to be transmitted is converted into light to be output to the light transmission path 3. An electric signal is converted to an optical signal by applying it directly to a semiconductor laser diode or the like serving as the signal light source or by modulating the light oscillated from the signal light source by an external modulator provided behind the signal light source before it is output to the light transmission path 3.
In the light receiving station 4, the optical signal 2 propagated through the light transmission path 3 is converted into an electric signal by a photoelectric converter, such as a photo diode, and the information transmitted from the light transmitting station 1 is demodulated, whereby the information is read.
As shown in FIG. 8, in the Raman amplifier 5, oscillation light from a pump light source 7 is input to a Raman amplification medium 31 through an optical coupler 6 from an end portion of the Raman amplification medium 31 constituting a part of the light transmission path 3 to cause Raman scattering in the Raman amplification medium 31, thereby effecting Raman amplification on the optical signal 2. Usually, the wavelength of the oscillation light to be selected is shorter than the wavelength of the optical signal 2 from the light transmitting station 1 by approximately 20 to 200 nm.
The Raman gain due to the Raman amplification in the Raman amplification medium 31 allows light amplification over a wide range. However, it does not allow uniform amplification over a wide signal band, exhibiting wavelength properties in amplification gain. Thus, to effect uniform Raman amplification over a wide signal band, a plurality of pump light sources of different wavelengths from each other are used in order to adjust the intensity of the pump lightwaves such that Raman gain is uniform over the signal band.
As described above, by adjusting each of a plurality of pump lightwaves, it is possible to adjust the optical signal such that the amplification gain properties are fixed over the entire signal band. However, when changing the once uniformly adjusted gain to some other gain, it has conventionally been general practice to individually readjust the intensity of the oscillation light output from each of the light sources over all the light sources.
However, the above method in which the intensity of oscillation light of a plurality of light sources is adjusted has a problem in that the gain of a specific signal band is not adjusted by adjusting one of the plurality of light sources; it involves a change in the gain of a still wider signal band including the specific signal band, so that it is not easy to adjust the gain to a desired value.